This relates generally to the formation of integrated circuits and, particularly, to the formation of components such as interconnects using a copper damascene process.
Semiconductor processing may incorporate many fast switching transistors into an integrated circuit. One consequence of a large number of transistors on one integrated circuit is an increase in power consumption. One technique for increasing speed, while reducing power consumption, is to replace the traditional aluminum or aluminum alloy interconnects found on integrated circuits with a metal, such as copper, which offers lower electrical resistance. Reducing resistance may cause electrical signals to propagate more quickly through the interconnect pathways on an integrated circuit.
Furthermore, because the resistance of copper is less than that of aluminum, the cross-sectional area of copper interconnects may be smaller than those used with aluminum interconnects without incurring increased signal propagation delays based on the resistance of the interconnect. Additionally, because the capacitance between two electrical nodes is a function of the overlap area between those nodes, using a smaller copper interconnect line may result in a decrease in parasitic capacitance. In this way, replacing aluminum based interconnects with copper based interconnects may reduce resistance and capacitance for a given dimension of interconnect.
Thus, the use of copper, compared to aluminum, may have electrical advantages including lower resistance per cross-sectional area, the ability to provide for reduced parasitic capacitance, and greater immunity to electromigration. For these reasons, semiconductor manufacturers may find it desirable to include copper in their products. They may use copper as interconnects at various levels, including the formation of vias or contacts to integrated circuit transistor source drains.
Copper is difficult to integrated into semiconductor process fabrication because copper can adversely affect the performance of metal oxide semiconductor field effect transistors. If copper migrates or diffuses into the transistor areas of an integrated circuit, adverse consequences may result. Therefore, copper diffusion barriers are used to isolate the copper from transistor areas. Additionally, unlike aluminum based interconnect systems which are formed by a substractive etch process, copper interconnects are typically formed by damascene metal processes. In the damascene process, a trench is formed in a first layer and a metal layer is formed over the first layer including the trench. Excess metal is then processed off, leaving individual interconnect lines in the trenches. The removal of excess copper is typically accomplished by chemical mechanical planarization or polishing.